A rapid and accurate method of identifying dermatophytes, the fungi which cause dermatophytoses, is needed. Because of their pleomorphic behavior many dermatophyte mutants present difficult taxonomic problems for both the clinical and general mycologist. Various methods of identification have been employed to delimit species and genera. Of the molecular approaches suggested, gel electrophoresis of proteins in polyacrylamide gels appears to be one of the most promising based on its sensitivity and the relative ease with which it can be accomplished. Certain evidence gained from the study of various organisms indicates that electrophoresis of isozymes may serve as markers for determining dermatophyte species in both their normal and mutant forms. The major objective of this study is to determine the variation in the electrophoretic isozyme patterns of dermatophyte species in the genus Microsporum and to subject this variation to analysis to determine the evolutionary relationships and genetic distance between the species. The initial stage of this program began with a careful study of the variation in the Microsporum gypseum complex (sensu lato), an imperfect species complex with three perfect stages, viz. Nannizzia gypsea, N. incurvata, and N. fulva. This study formed the basis for comparisons with other species and genera of dermatophytes and other soil fungi.